1. Field of the Invention
This invention relates to a construction machine provided with a hybrid system that drives a hydraulic pump by an engine and a motor generator.
2. Description of the Related Art
In construction machines with a hybrid system of this type applied thereto, that is, in so-called hybrid construction machines, there is known a technology that assists the power of an engine by allowing a motor generator to perform powering with electric power of an electrical storage device when a load associated with an operation of a vehicle body is large but charges the electrical storage device by allowing the motor generator to regenerate with the engine when a load associated with an operation of the vehicle body is small.
As another one of conventional technologies on hybrid construction machines, there is known a technology that includes an engine control means and a motor generation control means. The engine control means controls an operation of an engine, and the motor generation control means controls an operation of a motor generator. The engine control means allows the engine to operate with droop characteristics that its torque increases at a certain constant rate as its rotational speed decreases. The motor generation control means gives a target rotational speed to the motor generator, whereby the motor generator is allowed to operate under rotational speed control that performs control to allow the motor generator to generate a torque such that the rotational speed of the engine coincides with a target rotational speed (see, for example, JP2003-28071A).
Described specifically, according to the hybrid construction machine of this conventional technology, the motor generation control means gives a rotational speed, which corresponds to a region of high fuel economy and low emission in the operation characteristics (rotational speed-torque characteristics) of the engine, as a target rotational speed to the motor generator, so that the motor generator is allowed to operate primarily under rotational speed control. By the engine control means, the engine is allowed to produce at this time a torque corresponding to the controlled rotational speed according to the droop characteristics. It is, therefore, possible to always keep the rotational speed and torque of the engine in the region of high fuel economy and low emission even if the load associated with an operation of the vehicle body varies.